Disposable products made from papermaking fibers, either exclusively or in conjunction with various synthetic fibers, often serve as substitutes in both the home and in industrial shops for conventional cloth wipers and towels. Disposable paper-containing products such as wipers, baby wipers, food service wipers, feminine products, and other similar products should closely simulate cloth in both consumer perception and performance. Such products, including paper towels, industrial wipers, and other wiping products, are engineered to have as many cloth-like properties as possible.
For example, paper wiper products should exhibit good bulk, have a soft feel, have adequate strength even when wet, have good stretch characteristics, and resist tearing. These products should also be highly absorbent and be abrasion resistant.
Moreover, such products should not deteriorate in the environment in which they are used, regardless of whether they are used to absorb water or other types of liquids. In other words, in order to function as wipers to absorb liquids other than water, such products should exhibit a certain degree of solvent tensile strength. Solvent tensile strengths are machine direction and cross-machine direction strengths of a product that exist after the product has been exposed to, or wetted with, various solvents other than water. A paper wiping product that exhibits an acceptable solvent strength will generally maintain sufficient structural integrity even when wetted with a particular solvent to allow it to be used for its intended purpose of wiping and/or absorbing various liquids.
Typical solvents used for testing the solvent strength of products include methyl ethyl ketone, isopropyl alcohol, turpentine, and diesel fuel. If the wiping products retain sufficient dimensional strength after being soaked in solvents such as these, then such products are generally acceptable as heavy-duty wiper products.
In the past, many attempts have been made to enhance certain physical properties of disposable wiping products. Unfortunately, however, when steps are taken to increase one property of a wiping product, other characteristics of the product may be adversely affected. For instance, in cellulosic-based wiping products, softness is typically increased by reducing cellulosic fiber bonding within the paper product. Inhibiting fiber bonding, however, usually adversely affects the strength of the paper web.
One method that has been employed to reduce the stiff papermaking bonds is to crepe the paper from a drying surface with a doctor blade, which disrupts and breaks many of the interfiber bonds in the paper web. Other methods reduce these bonds by preventing formation of the bonds, rather than breaking them after they are formed. Examples of these other methods are chemical treatment of the papermaking fibers to reduce their interfiber bonding capacity before they are deposited on the web-forming surface, use of unrefined fibers in the slurry, inclusion into the slurry of synthetic fibers which are unable to form papermaking bonds, and use of little or no pressing of the web to remove the water from the paper web after it is deposited on the web-forming surface. This latter method reduces formation of bonds by reducing close contact of the fibers with each other during the forming process. Although these methods successfully increase the softness of paper webs, they result in a loss of strength in the web.
Attempts to restore the strength lost by reduction of papermaking bonds have included the addition to the web of bonding materials that add more strength than stiffness to the web. Such bonding materials may be added to the aqueous slurry of fibers and deposited on the web-forming surface along with the fibers. This method is commonly referred to in the industry as xe2x80x9csaturation bondingxe2x80x9d. With this method, the bonding material can be distributed generally evenly throughout the web to avoid the harshness which may accompany concentrations of bonding material. However, this method has the disadvantage of reducing the absorbency of the web by filling the pores between the fibers with bonding material.
Another method which has been used to apply bonding material to the web is to apply the bonding material in a spaced-apart pattern to the web. This method of applying the bonding materials to the webs in various pattems, typically through the use of rollers or the like, is known in the industry as xe2x80x9cprint bondingxe2x80x9d. Printing a bonding material, or adhesive, onto webs in various patterns results in a product where binder is applied only at localized areas defined by the particular roller pattern being utilized. In products made by this method, the majority of the web surface does not contain the absorbency-reducing bonding material. Print bonding is a method to be contrasted with other bonding methods such as the above-described saturation bonding method which results in a web that is impregnated with a bonding material substantially continuously over its entire surface.
In contrast to synthetic fiber-only nonwoven webs, webs made entirely or principally from cellulosic fibers require print bonding areas to be relatively close together because such cellulosic papermaking fibers are typically very short. The fibers are generally less than one-quarter of an inch long. Thus, it has been thought that to apply sufficient bonding material in a pattern to a paper web to the degree necessary to bond each fiber into the network would result in a harsh sheet, having poor softness characteristics, particularly in the areas where the bonding material is located. Various methods have been developed to enhance the softness characteristics of sheets where the bonding material is highly concentrated. Some of these processes that have proved to be successful in producing paper towels and other wiping products are disclosed in U.S. Pat. No. 3,879,257 to Gentile, et al., which is incorporated herein by reference in its entirety. In Gentile, et al., processes are disclosed for producing soft, absorbent, fibrous webs having a laminate-like structure that are particularly well suited for use as wiping products.
The fibrous webs disclosed in Gentile, et al. are made from a fibrous web formed from an aqueous slurry of principally lignocellulosic fibers under conditions which reduce interfiber bonding. A bonding material, such as a latex elastomeric composition, is then applied to a first surface of the web in a spaced-apart pattern. In particular, the bonding material may be applied so that it covers from about 50 percent to about 60 percent of the surface area of the web. The bonding material provides strength to the web and abrasion resistance to the surface. Once applied, the bonding material may penetrate the web typically from about 10 percent to about 40 percent of the thickness of the web.
The bonding material is then similarly applied to the opposite side of the web for further providing additional strength and abrasion resistance. Once the bonding material is applied to the second side of the web, one side of the web is brought into contact with a creping surface. The web adheres to the creping surface according to the pattern to which the bonding material was applied. The web is then creped from the creping surface with a doctor blade, which disrupts the fibers within the web where the bonding material is not disposed, thereby increasing the softness, absorbency, and the bulk of the web.
In an embodiment disclosed in Gentile. et al., each side of the paper web is creped after the bonding material has been applied to the side. Gentile et al. also discusses the use of chemical debonders to treat the fibers prior to forming the web in order to further reduce interfiber bonding and to increase softness and bulk.
Various other print bonding processes are also known in the art. A common denominator among such processes is that they employ a bonding material that usually comprises a latex elastomeric material. Typically, the use of such bonding materials is one of the most costly raw materials expenditures involved in the formation of paper-containing wiping products. In fact, where two printing (or latex bonding) processes are required, as in the double-print/double crepe processes disclosed in some embodiments of Gentile et al., the costs associated with producing soft, absorbent products can be very high.
Various water-borne epoxy resins and water-borne urethane resins have been utilized in a variety of instances in the past. For example, water-borne epoxy resins have been utilized in forming packaging materials and as adhesives binders for various ink formulations. A water-based polyurethane resin was disclosed in U.S. Pat. No. 5,656,701 to Miyamoto et al. as being used either as a binder for water-based printing inks or as a water-based laminating adhesive in conjunction with various resins such as low-density polyethylene, ethylene-vinyl acetate copolymer and polypropylene. Miyamoto et al., however, only describes the use of water-based polyurethanes as laminating adhesives for various plastic films produced from materials such as polyolefins, modified polyolefins, polyesters, nylons, and polystyrenes.
The prior art, however, is deficient in demonstrating the use of water-borne epoxies or water-borne urethanes in connection with standard print bonding adhesives in the context of print bonding fibers together to form paper-containing webs. The present invention provides that teaching.
The present invention recognizes and addresses some of the foregoing drawbacks, and deficiencies of prior art constructions and methods.
The present invention may accomplish its intended results by employing water-borne epoxies and/or water-borne urethanes as a replacement for at least a portion of the bonding materials utilized in various print bonding processes. The present additive may operate in conjunction with standard print bonding adhesives such as ethylene vinyl acetates to bond fibers together in a pulp-containing web.
More specifically, the present invention may involve the formation of a cellulosic-containing web and then printing a pattern onto at least one surface of the paper-containing web with a bonding material containing water-borne epoxies and/or water borne urethanes. The other surface may, if desired, then be printed with a similar bonding material in the same or another pattern. In order to enhance the properties of the web, the web may then be pressed to a creping surface where it is dried before being creped from the creping surface with a doctor blade, or comparable creping knife, thereby resulting in a product having increased softness, absorbency, and bulk, with a high amount of strength and elasticity. If desired, a second creping process may be utilized either after application of the bonding material to the first surface or after the bonding material has been applied to both surfaces.